California and Bay Area cities must start planning now for new and costly systems to control increasing runoff from urban storms, springtime floods from swollen rivers and rising sea levels as they invade lowlands, all as a result of global warming, climate scientists and water experts warn.

Climate change, they say, will result in thinner winter snowpacks in the Sierra and other Western mountains. As snowpacks melt earlier each spring, the meltwater will increase river flows and raise new threats of floods. Even a small rise in sea levels could threaten cities and farmland in low-lying areas, like the Delta and Silicon Valley.

New urban systems to handle winter storm runoff, new designs for dams and flood control structures, and higher dikes and levees around lands that even now lie below sea level will be needed, the scientists argue.

"The challenge is daunting," said Paul C. Milly of the U.S. Geological Survey, who led an international research team reporting in the journal Science on Saturday. "Patterns of change are complex, uncertainties are large; and the knowledge base changes rapidly."

Another report on climate change by a group of researchers headed by Tim P. Barnett of the Scripps Institution of Oceanography in La Jolla warns of "a coming crisis in water supply for the western U.S.," largely due to what the scientists call human-caused climate changes. This report is published Friday in the online version of Science.

Those changes, according to Barnett's team, have been altering the West's river flows, water temperatures and snowpacks for 50 years, according to records the team has analyzed. In California, runoff from the Sierra is starting earlier, while droughts already threaten Arizona, the scientists say.

The report by Milly's team focuses on policy and warns that because of climate change, the planners who design dams and flood control projects can no longer rely on records of orderly variations in recent past climates but must look ahead to an era when variations in the pace of warming will be rapid and unpredictable.

And that will mean rapidly changing seasonal averages and extremes of snow, rainfall, river flow and floods year-by-year, they say.

"You're in an area with substantial vulnerability to the impact of global warming," he said. "So with more rain in coming years, a diminishing snowpack and more danger of floods, you'll have your work cut out to start planning now."

Peter H. Gleick, a leading water resources expert and president of Oakland's Pacific Institute, was not part of either research group but said both teams are correct in warning that planners of dams, canals and systems for controlling floods and runoff must consider even more strongly than ever "the undeniable prospect of global warming as they design new facilities."

"Our sewers are 70 years old on average, and we have 60 to 70 miles of brick sewers that were built in the 1890s and are leaking badly," he said. "The system is falling apart. To meet the problems that are bound to come with more frequent and intense storms the scientists predict from global warming - that calls for imaginative solutions, like capturing storm waters for irrigation and other green uses."

Stephen H. McDonald, a partner with Carollo Engineers, a national firm that consults on water projects for many California cities, said his company already is helping San Francisco plan new systems to cope with surges of runoff from future heavy storms. And Silicon Valley towns will be designing new dikes and levees to protect against rising sea levels.

A $5.4 billion bond issue for dams and water conservation issues passed by California voters in 1986 includes $1 billion for "integrated water management infrastructure" to cope with problems posed by a warming climate, said Jenine Jones, an engineer and interstate resources manager at the state Department of Water Resources. "One of the elements we'll have to deal with in planning future work on California dams will be the best models available of ongoing climate change," she said.

Milly, lead author of the Science policy report, said the best global warming models indicate, first, that annual runoff everywhere will come earlier from diminishing snowpacks in the mountains, and second, that runoff totals will diminish sharply.

"A third would be sea-level rise and all that means for the delta, but let's focus ourselves here," he said in an e-mail message. "Planners must develop methods to grapple with increased hydrologic uncertainty in the design of systems to balance water supply and demand and to protect lives and property."

In the Science report on global warming and water, Barnett and his colleagues said that two-thirds of the measured climate change in the past 50 years has been human-induced - a conclusion Milly argued is understated.

But Barnett maintained that models of warming in the future mean "a coming crisis in water supply for the Western United States." And in an interview he put it more dramatically: "We're headed for a train wreck," he said.